In the field of the integrated circuit technology, the mobility of the electronic component is limited by the physical property of the carrier. In the prior art, it has disclosed that the strained silicon is applied on the metal oxide-semiconductor field-effect transistors (MOSFETs) in order to modify the physical property of the devices and increase the speed of MOSFETs. The silicon is typically formed on the relaxed silicon-germanium buffer layers. Because of the different lattice constants of silicon and germanium, wherein the lattice constant of germanium is 4% larger than that of silicon, the silicon would be strained by the tensile stress provided from the silicon-germanium buffer layers. The silicon-germanium buffer can be formed on the substrate of silicon-on-insulator (SOI) or on the traditional substrate of silicon. No matter the silicon-germanium buffer is formed on the substrate of SOI or on the traditional substrate of silicon, the speeds of P-type MOSFETs and N-type MOSFETs will be improved and increased.
If the strained silicon is directly formed on the traditional silicon wafer, the relaxed silicon-germanium buffer of the substrate has to be formed on the traditional silicon wafer first. Some defects would be performed due to the large difference between the lattice constant of the relaxed silicon-germanium buffer and that of silicon wafer. In order to minimize such kind of defects, the graded relaxed silicon-germanium buffer with increasing concentration of the germanium, where the thickness of the relaxed silicon-germanium buffer is around 0.5 to 2 μm, will be grown on silicon substrate first beneath the relaxed silicon-germanium buffer with a constant concentration of germanium, then the thin layer of the strained silicon layer is formed on the top of the buffer layer by the epitaxy growth. In all kinds of the methods applied for forming the blanket strained silicon in the prior art, the relaxed silicon-germanium buffer has to be formed on the substrate first. The steps of forming the relaxed silicon-germanium buffer on the substrate will induce the generateon of the dislocation at the interface of the strained silicon layer and the silicon-germanium buffer, and the generateon of the dislocation will decrease the quality of the strained silicon.
The mechanical method for providing the strain on the substrate is the four point bending method disclosed by Jeffery C. Shuling (as shown in Jeffery C. Shuling et al, IEEE Sensors Journal, Vol. 1, No. 1, pp. 14-30). The first step of the four point bending method is to fix the whole wafer with two points of the substrate, and then the strain on the wafer is induced by applying the stress with the other two points of the substrate. Generally speaking, the method is applied for the adjustment of the stress on the material of piezoresistance. Hitachi company in Japan and the research groups of MIT have used this method to process the strains of the components in the applications of MOSEFTs. Although the four point bending method is convenient and easy to be processed on the strained substrate, the strained substrate can't sustain the strain for a long time.
Therefore, the present invention provides a method for maintain the strained silicon layer for a long time, and the method with a mechanically strained silicon is capable of enhancing the speed of integrated circuits or devices so as to overcome the disadvantages of the prior art described above.